The present invention relates to the use of partial glycerides as lubricants in drilling fluids for geological exploration.
It is known that drilling fluids for sinking wells in rock and bringing up the rock cuttings are flowable systems thickened to a limited extent which may be assigned to any of the following three classes: purely aqueous drilling fluids, oil-based drilling fluids, which are generally used as so-called invert emulsion fluids, and preparations of the w/o emulsion type in which the aqueous phase is heterogeneously distributed as a fine dispersion in the continuous oil phase. The third class of known drilling fluids is built up on water-based o/w emulsions, i.e. on liquid systems which contain a heterogeneous, finely disperse oil phase in a continuous aqueous phase.
In addition to the basic constituents of a drilling fluid, that is to say water and/or oil, such systems also contain a large number of further constituents which are essential to their performance properties. These include, for example, weighting agents, generally barium sulfate (xe2x80x9cbaritexe2x80x9d) which imparts the necessary density to the fluid. Water-soluble salts, generally calcium chloride, are also added in order to prevent an osmotic compensation between the formation water and the drilling fluid. To obtain stable emulsions, suitable emulsifiers are generally also added. Corrosion inhibitors, viscosity-regulating additives, fluid loss additives, alkali reserves and also lubricants may also be present in the fluids.
The class of pure water-based systems is the oldest in the historical development of drilling fluids. However, their use is attended by such serious disadvantages that, hitherto, only limited application has been possible for technically demanding drilling operations. Above all, the interaction of the water-based drilling fluids with the water-sensitive layers of rockxe2x80x94more particularly corresponding layers of clayxe2x80x94to be drilled leads to unacceptable interference with the drilling process.
Very recently, however, a fairly old proposal has been taken up again and, even in highly sensitive shale formations, can lead to adequate stability where purely water-based drilling fluids are used. It involves the use of corresponding systems based on soluble alkali metal silicates which are also known as waterglasses or waterglass-based systems, cf. for example the seminar on xe2x80x9cThe Prevention of Oil Discharge from Drilling Operationsxe2x80x9d held in public in Aberdeen on Jun. 18/19, 1996 (organized by IBC Technical Services, London) and, in particular, the publications appearing in this connection by M. Eigner entitled xe2x80x9cField Trials with a Silicate Drilling Fluid in Shell-Exproxe2x80x9d and by I. Ward and B. Williamson entitled xe2x80x9cSilicate Water Based Mudsxe2x80x94a Significant Advance in Water Based Drilling Fluid Technologyxe2x80x9d.
However, the use of purely water-based drilling fluids, particularly of the last-mentioned type, makes the presence of components with a lubricating effect desirable. A large number of lubricants are known for use, in practice. These include mineral oils, animal and vegetable oils and esters. The increasingly stricter regulations with regard to the biodegradability of drilling fluids and their constituents are gradually restricting the use of the otherwise particularly suitable mineral oils. At the same time, there is a growing interest in alternatives with better biodegradability, in particular esters. EP 0 770 661, for example, describes esters of monocarboxylic acids with monohydric alcohols as suitable lubricants for water-based drilling fluid systems. However, only a 2-ethylhexyl oleate is actually mentioned as a suitable lubricant for silicate-containing aqueous fluids. Applicants"" DE 196 47 598 describes C12-30 fatty alcohols and a mixture thereof with fatty acid esters as suitable lubricants for purely water-based silicate-containing drilling fluids. Triglycerides of fatty acids are also mentioned as esters. It is known that a particularly pronounced lubricating effect is attributed in particular to carboxylic acid esters for the purpose of geological exploration, use being made of this in many ways. However, their use in water-based systems and above all in comparatively highly alkaline waterglass systems can lead to considerable difficulties. Ester cleavage can result in the formation of components with a marked tendency to foam which then introduce unwanted problems into the fluid systems. The above-mentioned publication by I. Ward et al. expressly refers to this difficulty.
Sulfonates of vegetable oils, in particular soya oil sulfonate, are also used as lubricants in practice. Soya oil sulfonate can be used in water- and oil-based systems, but shows significant foaming, especially in water-based fluids, which restricts its usefulness.
Accordingly, the problem addressed by the present invention was to provide a lubricant for both water- and oil-based drilling fluids which would not have the above-mentioned disadvantages. In addition, the lubricant would even be usable at low temperatures which can occur, for example, in drilling fluids that are used and stored in arctic regions. It has now been found that certain selected fatty acid partial glycerides have the desired properties.